A fresh look at NASA data suggests that a robotic mission uncovered microbial life on Mars—more than 30 years ago.

In 1976 NASA sent two space probes, Vikings 1 and 2, to Mars to determine whether life exists on the red planet. The probes carried three experiments specially designed for the task, one of which was called the Labeled Release (LR) apparatus.

The LR experiment worked by scooping up a bit of Martian soil and mixing it with a drop of water that contained nutrients and radioactive carbon atoms.

(Related: "Mars Soil Resembles Veggie-Garden Dirt, Lander Finds.")

The idea was that if the soil contained microbes, the life-forms would metabolize the nutrients and release either radioactive carbon dioxide or methane gas, which could be measured by a radiation detector on the probe.

A number of control experiments were also performed, including heating some Mars soil samples to different temperatures and isolating other samples in the dark for months—conditions that would kill microbes that are photosynthetic or that rely on photosynthetic organisms for survival. These control samples were also mixed with the nutrient solution.

To the delight of many biologists at the time, the LR experiment came out positive for life, and the control experiments came out negative.